The present invention relates generally to a variable capacitor and more specifically to a three terminal semiconductor varactor, the capacitance of which is controlled by a field effect portion of the device via a gate terminal.
Capacitors are basic elements in any electronic circuit, especially in circuits involving AC signals. Filter circuits, in particular, utilize capacitors and inductors to perform tuning and filtering functions. To minimize the size of circuits, it is desirable to fabricate the circuit as an integrated circuit. In such applications, it is necessary to vary the RF parameters of the circuit, which may be done by varying the value of the inductors or the capacitors in the circuit. However, an external inductor is often used in conjunction with an integrated capacitor, as it is difficult to incorporate an inductor into an integrated circuit, depending upon its value. Thus, to achieve circuit tunability, a capacitor is often the component selected to be varied.
Further, it is also known to form an integrated capacitor as a simple pn junction device or diode, often referred to as a variable capacitance diode or varactor diode. A reverse biased diode acts as a capacitor because the free-charge depletion region of a pn junction widens with the application of an increasing reverse bias voltage. Since the surfaces of the free-charge region represent the effective "plates" of a capacitor, the application of increasing reverse bias voltage will cause a decrease in the capacitance.
However, such integrated diode capacitors have significant limitations. To vary the capacitance, the DC voltage or reverse bias voltage of the diode capacitor is varied. This change in DC voltage affects other components in the circuit and causes a change in the optimal setting of other circuit components. For example, an inductor used in such a circuit is typically selected to be optimized for a predetermined DC voltage. A change in the DC voltage to vary the reverse bias voltage and, hence the capacitance of the capacitor, will have an adverse impact on the circuit. Thus, only small variations can be made.
Another limitation of such integrated diode capacitors is that the range of capacitance is severely limited by the useable range of reverse bias voltage. A standard curve (not shown) representing reverse bias voltage (x-axis) as a function of capacitance (y-axis) of the diode capacitor would indicate that capacitance drops off sharply as the magnitude of the reverse bias voltage increases, and levels out with further increasing reverse bias voltages, until breakdown occurs. Thus, to obtain a wide range of capacitance values, the voltage must be varied alone the steep portion of the curve. Accordingly, only a small voltage range is available for affecting the value of capacitance and therefore, the range of capacitance is limited and very difficult to accurately control.
Accordingly, it is an object of the present invention to provide a novel tunable semiconductor capacitor to substantially overcome the above-described problems.
It is an object of the present invention to provide a novel tunable semiconductor capacitor, the capacitance of which is electronically controllable.
It is another object of the present invention to provide-a novel tunable semiconductor capacitor, the capacitance of which is electronically controllable while maintaining a substantially constant reverse bias voltage.
It is yet an object of the present invention to provide a novel tunable semiconductor capacitor formed from capacitors integrally coupled within semiconducting material where one capacitor is reverse biased and the other capacitor is slightly forward biased.
It is a further object of the present invention to provide a novel tunable semiconductor capacitor wherein one capacitor is formed having a field-effect layer configured to provide a variable capacitance.
It is also an object of the present invention to provide a novel tunable semiconductor capacitor, the capacitance of which is varied by a voltage applied to a gate terminal of the field effect layer such that the capacitance is independent of a fixed reverse bias DC voltage level applied.